1. Field of the Invention
This invention relates to a method of producing a dense sintered silicon carbide body from polycarbosilane as a starting material, and more particularly, to a method of producing a strong sintered silicon carbide body which has high purity, high temperature flexural strength of about 2.4 to about 3.2 g/cm.sup.3 and a flexural strength of about 13 to about 45 kg/cm.sup.2.
2. Prior Art
The new ceramic engineering technology is capable of providing a sintered body of covalent solids such as SiC which can replace the conventional type sintered oxide ceramics. These conventional type oxide ceramics are obtained by sintering the powder of metallic oxides such as Al.sub.2 O.sub.3, SiO.sub.2, MgO, ZrO.sub.2, TiO.sub.2 alone or in mixtures. The establishing of sintering technology has been known for sometime along with the development of new materials sinterable into a dense body. As for the sintering of SiC, a dense sintered body of SiC has been obtained through a hot press process by using powdered SiC together with several percent of bonding agent such as aluminum, boron, metallic silicon, tungsten carbide, etc. It appears that such sintered SiC body is high in compressive strength, and excellent also in thermal shock resistance and oxidation resistance.
However, in the prior art processes, because components other than SiC, such as alumina, silicon, boron, free carbon, silicon nitride, tungsten carbide, etc. are included in the resulting sintered body as a bonding agent, the strength of the SiC body is reduced at elevated temperature which places restrictions on its use.
In U.S. Pat. No. 3,853,566, Prochazka has disclosed a production method of dense sintered silicon carbide articles by hot pressing submicron silicon carbide powder prepared by sedimentation techniques from commercially available silicon carbide powder (Norton E 277), and adding submicron B.sub.4 C powder as a sintering aid in a temperature range of 1900.degree.-2000.degree. and pressure range of 350-700 kg/cm.sup.2. The chemical and physical properties of the dense sintered silicon carbide developed by Prochazka are superior to those of dense sintered silicon carbide mentioned above. Unfortunately, this method is expensive and has difficulties in preparing and processing these submicron powders. That is to say, to obtain submicron silicon carbide powder, this method requires many process steps such as obtaining silicon carbide, suspension, sedimentation and separation processes, evaporation process, etc. Moreover, the yield by this method is so low that the sintered silicon carbide body is very expensive.
The methods of preparing a pure silicon carbide article from low molecular weight organosilicon polymers are disclosed in Japanese patent application Nos. 115965/1975 and 134122/1975, but these methods are not free from certain detriments either. That is to say, the low molecular weight organosilicon polymers disclosed in these Japanese patent applications melt or evaporate at low temperatures before forming silicon carbide during a firing process, producing a large quantity of exhaust gas by thermal decomposition of the polycarbosilane. Moreover, silicon carbide produced by the decomposition of polycarbosilane evaporates readily at sintering temperature. Accordingly, the decomposed gas and the volatile component remain within the sintered body in large quantities. Furthermore, blowholes are produced on the surface of the sintered body. The defects of the kind described substantially reduce the flexural strength and high temperature strength of the sintered body.